WASHINGTON -- Later this week in Paris, climate scientists will issue a dire forecast for the planet that warns of slowly rising sea levels and higher temperatures. But that may be the sugarcoated version.

Early and changeable drafts of their upcoming authoritative report on climate change foresee smaller sea level rises than were projected in 2001 in the last report. Many top U.S. scientists reject these rosier numbers. Those calculations don't include the recent, and dramatic, melt-off of big ice sheets in two crucial locations:

They "don't take into account the gorillas _ Greenland and Antarctica," said Ohio State University earth sciences professor Lonnie Thompson, a polar ice specialist. "I think there are unpleasant surprises as we move into the 21st century."

Michael MacCracken, who until 2001 coordinated the official U.S. government reviews of the international climate report on global warming, has fired off a letter of protest over the omission.

The melting ice sheets in Greenland and Antarctica are a fairly recent development that has taken scientists by surprise. They don't know how to predict its effects in their computer models. But many fear it will mean the world's coastlines are swamped much earlier than most predict.

Others believe the ice melt is temporary and won't play such a dramatic role.

That debate may be the central one as scientists and bureaucrats from around the world gather in Paris to finish the first of four major global warming reports by the Intergovernmental Panel on Climate Change. The panel was created by the United Nations in 1988.

After four days of secret word-by-word editing, the final report will be issued Friday.

The early versions of the report predict that by 2100 the sea level will rise anywhere between 5 and 23 inches. That's far lower than the 20 to 55 inches forecast by 2100 in a study published in the peer-review journal Science this month. Other climate experts, including NASA's James Hansen, predict sea level rise that can be measured by feet more than inches.

The report is also expected to include some kind of proviso that says things could be much worse if ice sheets continue to melt.

The prediction being considered this week by the IPCC is "obviously not the full story because ice sheet decay is something we cannot model right now, but we know it's happening," said Stefan Rahmstorf, a climate panel lead author from Germany who made the larger prediction of up to 55 inches of sea level rise. "A document like that tends to underestimate the risk," he said.

"This will dominate their discussion because there's so much contentiousness about it," said Bob Corell, chairman of the Arctic Climate Impact Assessment, a multinational research effort. "If the IPCC comes out with significantly less than one meter (about 39 inches of sea level rise), there will be people in the science community saying we don't think that's a fair reflection of what we know."

In the past, the climate change panel didn't figure there would be large melt of ice in west Antarctica and Greenland this century and didn't factor it into the predictions. Those forecasts were based only on the sea level rise from melting glaciers (which are different from ice sheets) and the physical expansion of water as it warms.

But in 2002, Antarctica's 1,255-square-mile Larsen B ice shelf broke off and disappeared in just 35 days. And recent NASA data shows that Greenland is losing 53 cubic miles of ice each year _ twice the rate it was losing in 1996.

Even so, there are questions about how permanent the melting in Greenland and especially Antarctica are, said panel lead author Kevin Trenberth, chief of climate analysis at the National Center for Atmospheric Research in Colorado.

While he said the melting ice sheets "raise a warning flag," Trenberth said he wonders if "some of this might just be temporary."

University of Alabama at Huntsville professor John Christy said Greenland didn't melt much within the past thousand years when it was warmer than now. Christy, a reviewer of the panel work, is a prominent so-called skeptic. He acknowledges that global warming is real and man-made, but he believes it is not as worrisome as advertised.

Those scientists who say sea level will rise even more are battling a consensus-building structure that routinely issues scientifically cautious global warming reports, scientists say. The IPCC reports have to be unanimous, approved by 154 governments _ including the United States and oil-rich countries such as Saudi Arabia _ and already published peer-reviewed research done before mid-2006.

Rahmstorf, a physics and oceanography professor at Potsdam University in Germany, says, "In a way, it is one of the strengths of the IPCC to be very conservative and cautious and not overstate any climate change risk."